1. Technical Field
The present invention relates to image forming systems comprising an image forming apparatus and a post processing apparatus that carries out the sheet post processing on sheets with image forming executed on them, and to image forming apparatus.
2. Description of the Related Art
In recent years, in companies and schools, image forming apparatuses (for example, printers) are proliferating that can accept simultaneously a plurality of print jobs (hereinafter called merely as “jobs”). As such an image forming apparatus, in Patent Document 1, a technology has been disclosed of including a setting section that sets the output priority level for each job, and if the priority level of a succeeding job is higher than that of a preceding job and also if the preceding job does not carry out sheet post processing operations, the preceding job under execution is interrupted and the succeeding job is executed.
In the following, the processing is explained briefly in a conventional image forming system having an image forming apparatus and a post processing apparatus. For example, as is shown in FIG. 8, in an image forming system in which two post processing apparatuses α and β are coupled to an image forming-apparatus, the output sheets (paper sheets) discharged by the image forming apparatus are subjected to sheet post processing operations in the respective post processing apparatus in units of a single sheet or in units of a booklet and are discharged. FIG. 8 shows an example in which the output sheets of Job A that carries out saddle stitching are discharged from a post processing apparatus α, and the output sheets of Job B that are not subjected to sheet post processing operations are discharged from the post processing apparatus β.
Taking Job A as the preceding job and Job B as the succeeding job, as is shown in FIG. 9(a), in Job A three sheets (images A-1, A-2, and A-3) are saddle stitched per copy set, in Job B four sheets (images B-1, B-2, B-3, and B-4) are output without any sheet post processing operations done on them (straight discharge). FIG. 9(b) shows the timing of sheet feeding (image formation) when outputting two sets each in Job A and Job B. In FIG. 9, at the time of carrying out the saddle stitching operation for Job A, since time is taken for the sheet post processing operations for each set in the post processing apparatus α, an example is shown in which a time gap between sets equivalent to three sheets is necessary (from the last sheet of one set to the first sheet of the next set). In this case, as is shown in FIG. 9(b), after the last sheet (A-3-1) of the first set of Job A has been fed, after leaving the necessary gap between sets (equivalent to 3 sheets), the first sheet (A-1-2) of the second set is fed (discharged to the post processing apparatus side). When the sheet feeding of Job A is completed, the sheet feeding of Job B is started. In this manner, at the time of executing a job involving sheet post processing operations, it is possible that time will be taken for sheet post processing operations in units of a set (unit of sheet post processing operations), and the output to the post processing apparatus side is being made after providing a prescribed gap in the sheet feeding interval during image formation.
Patent Document 1: Japanese Unexamined Patent Application Open to Public Inspection No. 11-188948
However, in a conventional image forming system, at the time of executing a job involving sheet post processing, the interval of sheet feeding during image formation has been made wider by a prescribed gap between sets, and during the period of this gap, the time gap is provided merely for obtaining the time necessary for carrying out sheet post processing and no processing is done on the image forming apparatus side, and hence there was the problem that the productivity gets reduced as an image forming system. In particular, when carrying out continuous outputs in large volume with jobs in the sheet post processing mode requiring such prescribed gap between sets, the cumulative time of the gap between sets during a job becomes extremely long.
Further, the technology disclosed in Patent Document 1 cannot be applied when the preceding job involves sheet post processing.
On the other hand, it is also possible to consider interrupting the preceding job and carrying out the succeeding job after interrupting the preceding job. In this case, the succeeding job is completed faster than when the execution of the succeeding job is started after the completion of the preceding job. However, in this case, after the completion of the succeeding job, it is necessary to execute the unpost processed part of the preceding job, and the sum of the time required for completing the preceding job and the time required for completing the succeeding job becomes either equivalent to or longer than when the execution of the succeeding job is started after the completion of execution of the preceding job, and hence the productivity of the image forming system does not improve.